The present invention describes a novel procedure for designing thin film filters using the technique of interleaving similar stacks. The resultant interleaved stacks can be used to make filters with extended ripple free passbands and reflectors with steep slopes.
This novel technique was developed because of the need to make IR reflective coatings with wide passbands in the visible portion of the spectrum. Two prior techniques for achieving this performance are the "2:1 stack" and stacks that require three materials with different indices of refraction typically designated as H=&gt;high, L=&gt;low and M=&gt;medium.
The 2:1 stacks are two material stacks with layers alternating between the H (high) and L (low) indices of refraction and having optical phase thicknesses such that either the H or L material is twice the optical phase thickness of the other. The disadvantage of this stack is that the visible passband is set by this stack and is narrow compared to the three material stacks or the interleaving technique to be described.
The three material stacks have passbands that are wide and whose edges are not defined by one stack. The passband is typically described by two stacks centered at somewhat different wavelengths. One stack is of the three material type and centered at the longer wavelength. The other stack is typically a standard quarter-wave stack (i.e. alternating H and L layers with quarter-wave optical phase thicknesses) and centered at the shorter wavelength. The disadvantage of these stacks is, three materials with the appropriate indices of refraction must be used or one of the three indices must be synthesized from the other two.
The novel technique of interleaving similar thin-film stacks eliminates the need to utilize stacks comprising three materials. The interleaving technique also achieves bandwidths in the passband similar to the three material technique.